1. Field of the Invention
This invention relates to cellular telecommunications systems and, more particularly, to a method and apparatus for providing fixed telephony service in a mobile cellular telecommunications system.
2. History of the Prior Art
In a cellular telecommunications system the user of a mobile station communicates with the cellular system through a radio interface while moving about the geographic coverage area of the system. The radio interface between the mobile station and cellular system is implemented by providing base stations dispersed throughout the coverage area of the system, each capable of radio communication with the mobile stations operating within the system. In a typical cellular system each base station of the system controls communications within a certain geographic coverage area termed a cell, and a mobile station which is located within a particular cell communicates with the base station controlling that cell. A cellular system is typically connected to the public switched telephone network (PSTN) or other similar landline phone system.
Radio technology has also for some time been used to provide a radio interface for fixed telephony service. In fixed telephony service, as opposed to mobile service, a subscriber remains in one location and does not move throughout the coverage area of the system providing the service. Noncellular systems providing fixed telephony services are generally referred to as Radio in the Local Loop (RLL) systems. In an RLL system, subscriber equipment usually comprises radio hardware having a radio interface to the RLL system and, an interface that accepts conventional telephone equipment through which the subscriber communicates with the RLL system. RLL systems are not capable of providing cellular functionality. As was the case for cellular systems, a typical RLL system is connected to the PSTN or other similar landline phone system. Preferably, the effect of using a radio interface is transparent and the subscriber does not notice that he or she is using a telephone which accesses the phone system through radio. Traditionally, RLL systems have been implemented mostly using analog radio systems.
A system operator desiring to provide fixed telephony service using RLL may implement the service using a system that is uniquely designed to provide only fixed telephony. As an alternative the system operator may implement the service using a mobile cellular telephone system to provide switching and radio access. If a digital cellular system is used this is referred to as digital wireless local loop (DWLL). If the system operator has an existing cellular phone system available the alternative may be preferred.
Implementation of fixed telephony services in an existing cellular system is typically done by providing the fixed subscriber with equipment differing from the equipment provided to a mobile subscriber. Since it is not necessary to provide portability to the degree necessary for a mobile subscriber, the fixed terminal is normally heavier and may be much larger than portable equipment. This type of terminal may be fixed in position, such as, for example, a pay phone is fixed to a structure. It is also possible that the fixed subscriber is provided a portable mobile station but the subscriber is restricted by agreement to remain within a certain fixed area of the cellular system while using the system. In this case the fixed subscriber may be restricted to a certain cell or smaller area within a cell. In both of the above cases, operators of a cellular system can provide simultaneous fixed and mobile service to different subscribers and allow easy upgrading of a subscriber's service from fixed to mobile.
One application of fixed telephony provided within a cellular system would be in developing countries where no landline telephone infrastructure exists.
Since a fixed subscriber does not move about the system, a fixed subscriber has a predictable effect on system radio channel allocation requirements. For example, a fixed subscriber will always access radio channels allocated to its assigned area. A fixed subscriber also uses less of certain switching resources than a mobile subscriber. For example, the fixed subscriber does not move from cell to cell so handoff switching resources are not required.
The predictability of the effect of fixed subscribers on radio channel allocation requirements allows a system operator to assign radio channels and equipment resources of a cellular system accordingly. For these reasons the system operator may charge a fixed subscriber a reduced rate as compared to a mobile subscriber.
In the above described fixed telephony within a cellular system, the system operator has an interest in assuring that a fixed subscriber remains fixed, i.e., remains fixed in position, or fixed within an assigned fixed subscriber area within the cellular system during use. If fixed subscriber services are implemented into the system with a minimum of system modification and fixed subscribers are registered in the system in a manner similar to and have similar equipment to mobile subscribers, the system operator may not always depend upon the lack of portability of the fixed terminal or the subscriber's adherence to an agreement to assure that the subscriber remains in place. In this type of implementation a fixed subscriber could conceivably modify a fixed terminal or break his agreement with the system operator and move out of his assigned fixed area. It would then be desirable to detect and prevent fixed subscriber movement from occurring at the outset rather than use system subscriber records to track down violators.
In mobile cellular systems the system keeps track of a mobile subscriber's position. In most systems this is done by registering the mobile subscriber in a location area that comprises one or more cells. When the subscriber moves to a new location area, registration in the new location area takes place. The registration is accomplished by the exchange of signals transmitted between the mobile subscriber's terminal and the base station, or base stations, controlling the newly entered location area. The registration process basically involves informing the mobile services switching center (MSC) controlling the newly entered location area of the mobile station's identity and then receiving a confirmation message from the system indicating that the system has registered the mobile station in the new location area.
One solution to the problem of inhibiting fixed subscriber movement in a cellular system would be to inhibit the movement of the fixed subscriber to a particular location area by inhibiting the registration process in all other location areas. However, the cells of a system and the location areas comprising these cells can be of considerable size. Inhibiting registration to a single location area would not prevent supposedly fixed subscriber from moving within that location area.
Another solution to the problem of restricting a fixed subscribers movement could be to inhibit the handover function for the fixed subscriber. The handover function is provided in most cellular systems and allows a subscriber engaged in a call to move from a cell under the control of one base station to a second cell under the control of a second base station, while maintaining communications with the system. If the handover function were to be disabled for a subscriber, it would achieve the effect of pinning a subscriber to one cell, the one cell being the cell within which a call was initiated. However, disabling the handover function would not prevent the subscriber from moving to another cell and initiating a new call. By doing so the subscriber could move from his assigned fixed area and use the system.
Also, in some systems it may be desirable to have a handover function for fixed subscribers. This would be the case, for example, in systems where there is an overlaid cell structure with smaller cells within larger cells, each sharing coverage areas. In this situation, when system traffic requires, it may be desirable to handover calls for a fixed subscriber from a overlaying cell to the smaller cell within the larger encompassing cell or vice versa. Having the handover function available for all subscribers within a system would also allow easy upgrading of services from fixed to cellular.
It would provide an advantage then to have a method and apparatus for providing fixed telephony in a cellular telecommunications system, that restricted a fixed subscriber using any type of subscriber station to an area smaller in size than a system location area. It would provide a further advantage if the method and apparatus did not require inhibition of the handover function for system subscribers, easing simultaneous provision of both fixed and cellular services within the same system. Such a method and apparatus would provide the benefits of fixed telephony without the problems associated with the existing methods of restricting a subscriber's movement.